This disclosure is directed to thermoplastic compositions, articles formed therefrom, and their methods of manufacture, and in particular laser weldable compositions comprising polyesters, methods of manufacture, and articles thereof.
Polymer articles having high thermal resistance, for example from 100 to 150° C., or higher, are desirable in a wide variety of applications, for example electronic devices, and parts for automobiles and other transports, such as ships and jets. There is a particular need for components parts for “under-the hood” applications in transports, for example sensor housings.
Component parts can be assembled into larger ones by processes such as gluing, ultrasonic welding, hot-plate welding, and vibration welding. Recently, the use of lasers to join component parts has become more widespread. In laser welding of two polymer parts by transmission welding, one of the polymer parts is substantially transparent to laser light to allow transmission to the welding interface, and the other polymer part absorbs sufficient laser light to generate heat for welding at the interface of the parts. External pressure can be applied to provide uninterrupted contact between the surfaces of the parts, and heat conduction between the parts results in the melting of the polymers in both the absorbing and the transmitting parts, to provide a weld at the interface. When laser light of near-infrared (NIR) wavelength is used for welding, the level of NIR transmission through the transparent part should allow sufficient laser-light density to arrive at the interface to facilitate effective and rapid welding. Otherwise, joining of the two parts by laser welding would be impractical or limited to slow scan speeds. It is desired that the cycle time for assembly of parts be as short as possible.
Poly(butylene terephthalate) (PBT) and PBT reinforced with glass fibers or mineral fillers can be used in numerous applications, especially in the automotive and electrical industry, owing to their excellent electrical resistance, surface finish, and toughness. A potential problem with welding materials based on crystalline or partially crystalline materials, such as PBT, however, is that such polymers can also partially disperse or scatter incoming radiation. Consequently, the extent of the laser energy arriving at the joining interface can be diminished, thereby reducing the adhesion between the parts to be welded. In particular, a reduction in weld strength for a given amount of laser energy applied to the article to be welded can result in a substantial increase in laser welding assembly cycle time.
Additionally, scattering effects are greatly enhanced when fillers such as glass fibers are present, especially when the laser transparent part thickness is greater than 1 millimeter. Therefore, the laser-welding of crystalline polymers, particularly glass-filled crystalline polymers, is restricted, if not impracticable in many cases.
Another drawback of laser welding is that internal scattering of the laser light in the laser-transparent polymer part can cause a temperature increase, especially in thick-walled parts, and can result in sink marks or burning along the weld line in the laser-transparent part. It is therefore beneficial to have high and consistent laser transparency across a range of thicknesses and processing conditions of the part to achieve consistent weld strengths.
One approach to increase laser transparency is to speed the rate of crystallization of the polymer using a chemical nucleant. This can occur by chemical reaction between the nucleating agent and polymeric end groups of PBT to produce ionic end groups that enhance the rate of crystallization. The addition of such chemical nucleants, however, can lower the molecular weight of the crystalline material and lead to unstable melt viscosity. Additionally, such chemical nucleants can substantially degrade many of the amorphous materials used in PBT blends, causing unstable melt viscosities and other undesirable defects such as splay and jetting (deformations due to turbulent flow).
It is therefore desired to achieve improved NIR transmission for laser-weldable thermoplastic compositions that provide heat resistance, especially compositions comprising glass fibers or other fillers.